The present invention relates to a semiconductor laser light emitting device, and particularly to a semiconductor laser light emitting device including a stack of group III nitride semiconductor films.
A semiconductor laser light emitting device of a type including a stack of semiconductor films made from group III nitrides each containing at least one kind selected from aluminum, gallium, indium, and boron (hereinafter, referred to as “group III nitride semiconductor light emitting device”) has been disposed as a useful component of various systems, and a system using such a semiconductor laser light emitting device as a light source is being realized.
The group III nitride semiconductor laser light emitting device is inferior to semiconductor laser light emitting devices of a type including a stack of semiconductor films made from other materials, for example, an AlGaAs based semiconductor laser light emitting device used for compact disks (CDs) and mini-disks (MDs) and an AlGaInP based semiconductor laser light emitting device used for digital versatile disks (DVDS) and bar-code readers in various points one of which is a transverse mode control.
The transverse mode is a light waveguide mode in the in-plane direction of a semiconductor film, and a method of controlling the transverse mode in a semiconductor laser light emitting device has been established. Referring to FIG. 13, there is shown a stacked semiconductor film 311. Now, assuming that a stacking direction of the stacked semiconductor film 311 is taken as a vertical direction; a direction within a plane of the stacked semiconductor film 311 and perpendicular to the length direction of a resonator including the stacked semiconductor film 311 as a main component is taken as a horizontal direction; and the length direction of the resonator is taken as a longitudinal direction, the above-described transverse mode is a light waveguide mode in the horizontal direction.
The mechanism of generating a horizontal light waveguide mode in a semiconductor laser light emitting device is generally classified into two mechanisms.
One light waveguide mechanism is obtained by a structure shown in FIG. 14A, in which the configuration of a semiconductor film 320 in the vertical direction is made constant between a current injection portion 321 and a current non-injection portion 322. In such a structure, a built-in differential refractive index Δn between a refractive index n1 of the current injection portion 321 and a refractive index n2 of the current non-injection portion 322 becomes zero (Δn=0), so that at the time of current injection, a distribution of refractive indexes in the horizontal direction as shown in FIG. 14B occurs and thereby a distribution of carriers as shown in FIG. 14C occurs. As a result, a light waveguide mechanism, called as a gain guide waveguide mechanism, is generated.
The other light waveguide mechanism is obtained by a structure shown in FIG. 15A, in which the configuration of a semiconductor film 330 in the vertical direction is made different between a current injection portion 331 and a current non-injection portion 332. In such a structure, a built-in differential refractive index Δn between a refractive index n1 of the current injection portion 321 and a refractive index n2 of the current non-injection portion 322 does not become zero (Δn=n1-n2≠0), so that at the time of current injection, a distribution of refractive indexes in the horizontal direction as shown in FIG. 15B occurs and thereby a distribution of carriers as shown in FIG. 15C occurs. As a result, a light waveguide mechanism, called as an index guide waveguide mechanism, is generated.
In general, the index guide waveguide mechanism is further classified into two mechanisms. One mechanism in which waveguide of light occurs if a real portion of the differential refractive index is larger than zero [(Δn-real) >0] is called a rear index guide type, and the other mechanism in which waveguide of light occurs if an imaginary portion of the differential refractive index is smaller than zero [(Δn-im)<0] is called a loss index guide type. To stably keep the transverse mode at a higher output, the loss index guide type semiconductor laser light emitting device is superior to the gain guide type semiconductor laser light emitting device. From the viewpoint of operational current, the real index guide type semiconductor laser light emitting device is superior to the loss index guide type semiconductor laser light emitting device.
The light waveguide mechanism can be more finely classified as follows: namely, at a value of Δn between a value realizing the gain guide type and a value realizing the index guide type, self-pulsation occurs, and such a mechanism realized at a weak value of Δn is called a weak index type self-pulsation waveguide mechanism.
Each of the commercial available semiconductor laser light emitting devices such as the AlGaAs based semiconductor laser light emitting device used for CDs and MDs and the AlGaInP based semiconductor laser light emitting device used for DVDs and bar-code readers adopts a transverse mode waveguide mechanism which is varied depending on its application by changing its film structure. More specifically, in these commercial available semiconductor light emitting devices, values of Δn are arbitrarily changed.
For example, for the purpose of preferentially reducing laser noise with lower power drive, the gain guide type is selected as the transverse mode waveguide mechanism, and for the purpose of most preferentially reducing laser noise, the pulsation type is selected as the transverse mode waveguide mechanism. On the other hand, for the purpose of preferentially stabilizing a radiation angle [(Far Field Pattern (FFP)] and an astigmatism of a laser beam with a high power drive, or for the purpose of preferentially reducing a drive current, the index guide type is selected as transverse mode waveguide mechanism.
From the above description, it becomes apparent that the transverse mode of the group III nitride semiconductor laser light emitting device is needed to be controlled, that is, varied depending on its application in accordance with the same manner as that adopted by the commercial available semiconductor laser light emitting devices such as the AlGaAs based semiconductor laser light emitting device used for CDs and MDs and the AlGaInP based semiconductor laser light emitting device used for DVDs and bar-code readers.
The group III nitride semiconductor laser light emitting device using a buried ridge structure, however, is difficult to control the transverse mode in the same manner as that adopted by the commercial available semiconductor laser light emitting devices. The reasons for this are as follows:
(1) In the group III nitride semiconductor laser light emitting device, the details of a buried layer have been not made clear.
(2) To control the transverse mode, it is required to examine a differential refractive index Δn between an effective refractive index n1 of a current injection region in the film stacking direction and an effective refractive index n2 of a current non-injection region in the film stacking direction; however, any group III nitride semiconductor laser light emitting device attempted to examine the differential refractive index Δn has been not disclosed. Accordingly, from the viewpoint of transverse mode control, it has failed to desirably design and produce a group III nitride semiconductor laser light emitting device.
(3) For example, Japanese Patent Laid-open No. Hei 11-214788 has disclosed a buried ridge type semiconductor laser light emitting device, wherein an insulating film or a semiconductor film is taken as a buried layer. In this document, however, a thickness d2 from an active layer and a current non-injection region has been not described. The thickness d2 is an important structural parameter for determining the differential refractive index Δn, and accordingly, unless the thickness d2 is made clear, the differential refractive index Δn cannot be determined. The invention disclosed in the above document, Japanese Patent Laid-open No. Hei 11-214788 is intended to achieve self-pulsation by adopting the weak index type waveguide mechanism; however, since the thickness d2 is not made clear, the differential refractive index Δn cannot be determined, and therefore, it fails to product a self-pulsation type semiconductor laser light emitting device.
(4) Even in related art inventions other than that disclosed in the above document, the relationship between the differential refractive index Δn and the transverse mode has not been made clear, and the transverse mode has been specified only by a material composition or a stripe width.
The reason for this is supposed to be due to the fact that a ridge structure in direct-contact with a metal film or an insulating film has been adopted in study and development of group III nitride semiconductor laser light emitting devices. In the ridge structure in direct-contact with a metal film, since leak failures frequently occur, the thickness d2 cannot be positively made small and thereby the differential refractive index Δn is unclear, with a result that the transverse mode cannot be examined on the basis of the differential refractive index Δn. Such a ridge structure causes another problem that a leak current outside the ridge structure becomes larger and thereby an operational current becomes larger.
In the group III nitride semiconductor laser light emitting device, the increased drive power causes an increase in heat generation, and at the worst case, makes it difficult to perform laser oscillation. The increased drive voltage also makes it difficult to specify a current injection width Wst of a current injection region, and thereby tends to degrade the stability of the transverse mode. This is one of factors of making it difficult to examine the transverse mode.
(5) On the other hand, in a structure buried in a power constriction region by using an insulating film, no leak failure occurs even if the thickness d2 becomes thinner, so that the differential refractive index Δn can be made sufficiently larger. In this structure, however, since the adhesion degree and film quality are uneven on both side surfaces of a ridge stripe, the transverse mode becomes unstable, thereby failing to examine the transverse mode. Further, since the refractive index of the power constriction region is fixed at a specific value and is not changed, the differential refractive index Δn is adjusted only by controlling the thickness d2; however, such a control of the thickness d2 exerts an effect on leak current, with a result that it becomes difficult to independently adjust the spread of a current and the spread of light-Accordingly, the production of the buried ridge structure using an insulating film is poor in advantage from the practical viewpoint.
(6) To solve the above-described various problems of the buried ridge structure using a metal or insulating film, a buried ridge structure using a semiconductor film may be preferably used. However, even in a semiconductor laser light emitting device having the buried structure using a semiconductor film, since a correlation between the differential refractive index Δn and the transverse mode is not clear, it is impossible to desirably select the transverse structure depending on the application of the device.